The Zimbabwean opposition leader, Morgan Tsvangirai, has told South Africa's president, Thabo Mbeki, that he is no longer fit to serve as the region's mediator in Zimbabwe's political crisis owing to a "lack of neutrality", and that "there will be no country left" if Mbeki continues to side with President Robert Mugabe.

The warning comes in a letter from the Movement for Democratic Change (MDC) leader to Mbeki made public just days after it was revealed that the South African president had written a four-page letter to George Bush demanding that the US president stop criticising Mugabe.

In his letter, Tsvangirai accuses Mbeki of colluding with Mugabe to play down the deepening political crisis, of blocking UN security council discussions on Zimbabwe and of trying to facilitate a controversial weapons delivery from China to the Zimbabwean military.

But some of the strongest criticism is over Mbeki's reaction to the escalating state-sponsored campaign of murder, violence and arrests against the opposition in the run up to the run-off presidential election between Mugabe and Tsvangirai at the end of this month. At least 50 people have been killed and thousands beaten.

The letter, dated May 13, accuses South Africa's president of ignoring evidence that Harare was planning the violence, including a leaked Zimbabwean military document outlining the strategy that Tsvangirai personally handed to Mbeki.

"When you started mediating, Zimbabwe still had a functioning economy, millions of our citizens had not fled to other countries to escape political and economic crisis, and tens of thousands had not yet died from impoverishment and disease. In fact, since the March 29 election, Zimbabwe has plunged into horrendous violence while you have been mediating. With respect, if we continue like this, there will be no country left," writes Tsvangirai.

"As you know, when MDC attempted to appeal to the UN security council to investigate and help stop the carnage, it was you, the so-called 'neutral' mediator, who blocked a possible road to resolution of the crisis."

Tsvangirai says Mbeki continued to act as if everything was normal, even after the Zimbabwean government blocked the release of poll results showing that Mugabe and his Zanu-PF party had lost.

"Your lack of neutrality became increasingly evident when I arrived to the Lusaka summit to see you and Mr Mugabe on television together proclaiming there is 'no crisis' in Zimbabwe," the letter says.

Tsvangirai also accuses the South African government of facilitating the delivery of weapons via Durban from a Chinese ship that was eventually turned away by dock workers and legal action.

"Not only have you been unable to denounce the well-documented post-election attacks on our people, but your government even played a role in Zimbabwean government procurement of weapons of repression ... and agree to allow passage of arms of war purchased by the same government through South African territory during the troubled post-election period," he writes.

The letter demands Mbeki step down as the Southern African Development Community mediator on Zimbabwe, as the MDC no longer has confidence in him.

His spokesman, Mukoni Ratshitanga, has denied knowledge of the letter even though the MDC says it has a receipt showing it was delivered to Mbeki's office. Ratshitanga has also denied knowledge of the letter to Bush revealed by the Washington Post last week in an article which quotes an unnamed US official as saying Mbeki told Bush to "butt out" of Zimbabwe.

But the US embassy in Pretoria confirmed that Mbeki's letter existed and was delivered to Bush.

Yesterday, the leader of a breakaway MDC faction, Arthur Mutambara, was arrested and charged over a newspaper column criticising Mugabe's handling of the economy, with inflation now above 1,000,000%, and accusing the security forces of abuses.